Organic light emitting devices (OLEDs) are light emitting devices that are comprised of several layers, in which one of the layers is comprised of an organic material that can be made to electroluminesce by applying a voltage across the device, C. W. Tang et al., Appl. Phys. Lett 51, 913 (1987). Certain OLEDs have been shown to have sufficient brightness, range of color and operating lifetimes for use as a practical alternative technology to LCD-based full color flat-panel displays (S. R. Forrest, P. E. Burrows and M. E. Thompson, Laser Focus World, Feb. 1995). Furthermore, since many of the organic thin films used in such devices are transparent in the visible spectral region, they allow for the realization of a completely new type of display pixel in which the red (R), green (G), and blue (B) emission layers are placed in a vertically stacked geometry to provide a simple fabrication process, a small R-G-B pixel size, and a large fill factor.
A transparent OLED (TOLED) which represents a significant step toward realizing high resolution, independently addressable stacked R-G-B pixels has been reported in International Patent Application No. PCT/US95/15790. This TOLED had greater than 71% transparency when turned off and emitted light from both top and bottom device surfaces with high efficiency (approaching 1% quantum efficiency) when the device was turned on. The TOLED used transparent indium tin oxide (ITO) as the hole-injecting electrode and a Mg-Ag-ITO layer for electron-injection. A device was disclosed in which the Mg-Ag-ITO electrode was used as a hole-injecting contact for a second, different color-emitting OLED stacked on top of the TOLED. Each device in the stacked OLED (SOLED) was independently addressable and emitted its own characteristic color through the transparent organic layers, the transparent contacts and the glass substrate, allowing the device to emit any combination of color that could be produced by varying the relative output of the red and blue color-emitting layers.
Thus, publication of PCT/US95/15790 provided the disclosure of an integrated OLED where both intensity and color could be independently varied and controlled with external power supplies in a color tunable display device. As such, PCT/US95/15790 illustrates a principle for achieving integrated, full color pixels that provide high image resolution, which is made possible by the compact pixel size. Furthermore, relatively low cost fabrication techniques, as compared with prior art methods, may be utilized for making such devices.
In such a stacked structure, one electrode layer is provided at the bottom of the SOLED stack and a further electrode layer is provided between each of the OLEDs in the stack and on top of the uppermost OLED in the stack. The bottom electrode layer is typically coupled to a ground reference and the intermediate and top electrode layers are coupled to either a positive or negative driving voltage.
With only one electrode layer being provided in the SOLED for coupling to a ground reference and with only one additional electrode layer being provided for each OLED in the stack, the problem arises of driving each of the OLEDs in the stack independently of each other. There is therefore a need for a means of driving such a configuration of stacked OLEDs or for alternative configurations of stacked OLEDs, such as disclosed in co-pending application having Ser. No. 08/792,050, filed Feb. 3, 1997.
Devices whose structure is based upon the use of layers of organic optoelectronic materials generally rely on a common mechanism leading to optical emission. Typically, this mechanism is based upon the radiative recombination of a trapped charge. Specifically, devices constructed along the lines discussed above comprise at least two thin organic layers separating the anode and cathode of the device. The material of one of these layers is specifically chosen based on the material's ability to transport holes (the "hole transporting layer") and the material for one of the other layers is specifically selected according to its ability to transport electrons (the "electron transporting layer"). With such a construction, the device can be viewed as a diode with a forward bias when the potential applied to the anode is higher than the potential applied to the cathode. Under these bias conditions, the anode injects holes (positive charge carriers) into the hole transporting layer, while the cathode injects electrons into the electron transporting layer. The portion of the luminescent medium adjacent to the anode thus forms a hole injecting and transporting zone while the portion of the luminescent medium adjacent to the cathode forms an electron injecting and transporting zone. The injected holes and electrons each migrate toward the oppositely charged electrode. When an electron and hole localize on the same molecule, a Frenkel exciton is formed. Recombination of this short-lived state may be visualized as an electron dropping from its conduction potential to a valence band, with relaxation occurring, under certain conditions, preferentially via a photoemissive mechanism. Under this view of the mechanism of operation of typical thin-layer organic devices, the electroluminescent layer comprises a luminescence zone receiving mobile charge carriers (electrons and holes) from each electrode.
The materials that are responsible for producing the electroluminescent emission are frequently incorporated into the OLED such that they also serve as the electron transporting layer of the OLED. Such devices are referred to as having a single heterostructure. However, alternatively, the electroluminescent material may be present in a separate emissive layer between the hole transporting layer and the electron transporting layer in what is referred to as a double heterostructure.
In addition to having the emissive material present as the primary material in the electron transporting layer, the emissive material may also be present as a dopant that is contained within a host material. Materials that are present as host and dopant are selected so as to have a high level of energy transfer between the host and dopant materials. In addition, these materials need to be capable of producing acceptable electrical properties for the OLED. Furthermore, such host and dopant materials are preferably capable of being incorporated into the OLED using starting materials that can be readily incorporated into the hole transporting layer or electron transporting layer of an OLED using convenient fabrication techniques.
Demonstration of efficient electroluminescence from vacuum deposited molecular organic light emitting devices has generated interest in their potential application for emissive flat panel displays. To be useful in low cost, active matrix displays, device structures which are integratable with pixel electronics need to be demonstrated. A conventional OLED is grown on a transparent anode such as ITO, and the emitted light is viewed through the substrate, complicating integration with electronic components such as silicon-based display drivers. It is therefore desirable to develop an OLED with emission through a top, transparent contact. A surface-emitting polymer-based OLED grown on silicon with a transparent ITO and a semitransparent Au or Al top anode has been demonstrated, D. R. Baigent, et al., Appl. Phys. Lett. 65, 2636 (1994); H. H. Kim, et al., J. Lightwave Technol. 12, 2107 (1994).
A similar integration of molecular OLEDs with silicon was achieved using a tunneling SiO.sub.2 interface, Kim et al. The tunneling interface, however, increases the device operating voltage, and can be avoided in structures such as for the recently reported transparent TOLEDs, V. Bulovic, et al., Nature 380, 29 (1996). G. Gu, Et al., Appl. Phys. Lett. 68, 2606 (1996), which can, in principle, be grown on a silicon substrate. The TOLED anode, however, forms the electrode contact which is in direct contact with the substrate, "the bottom contact", whereas for display drivers employing n-channel field effect transistors (NFETs), such as amorphous silicon NFETs, it would be desirable for the bottom contact of the OLED to be the cathode. This would require fabricating inverted OLEDs (IOLEDs), that is, devices in which the order of placing the sequence of layers onto the substrate is reversed. For example, for a single hererostructure OLED, the electron-injecting cathode layer is deposited onto the substrate, the electron transporting layer is deposited on the cathode, the hole transporting layer is deposited on the electron transporting layer and the hole-injecting anode layer is deposited on the hole transporting layer. Fabrication of IOLEDs would, thus, require that the ITO anode be sputter-deposited on relatively fragile hold-conducting organic thin film. Such sputter-deposition of the ITO can result in unacceptable degradation of the device operating characteristics.
It would be desirable if the hole injection characteristics of thge hole injecting layer could be improved. It has been shown that CuPc can serve as an efficient hole injection layer in conventional OLEDs, S. A. VanSlyke et al., Appl. Phys. Lett 69, 2160 (1996) and U.S. Pat. No. 4,720,432 and it had previously been established that PTCDA is an efficient hole transporting layer, P. E. Burrows et al., Appl. Phys. Lett. 64, 2285 (1994), V. Bulovic et al., Chem. Phys. 210, 1 (1996); and V. Bulovic et al. Chem. Phys. 210, 13 (1996); respectively. Furthermore, the use of PTCDA in a photodetector structure with an ITO electrode deposited on the film surface, F. F. So et al., IEEE.Trans. Electron. Devices 36, 66 (1989), has previously demonstrated that this material can withstand sputter-deposition of ITO with minimal degradation to its conducting properties.